The procedure for charging a lithium battery is conducted in two phases: a first constant-current phase, during which the charging current is maintained at a preset value and the voltage at the terminals of the battery increases progressively, until it reaches a preset voltage value and a second constant-voltage phase, in which the voltage is kept at said preset value whilst the charging current decreases progressively, until battery charging is completed, which is considered to have been achieved when the charging current reaches a preset value, equal, for example, to about 1/10 of the intensity of the charging current in the constant-current charging phase.
In the aforementioned charge procedure, the voltage applied to the terminals of the battery in the constant-voltage charging phase is a critical factor, because permitted variation in voltage with respect to said preset value is very small, of the order of 1%.
Voltage that is more than 1% below the preset charge voltage leads to a noticeable prolongation of charge time, with the further risk of an incomplete charge. If the charge voltage exceeds the preset value by more than 1% there is the risk of drastically decreasing the life of the battery and of irreversible damage to the battery, fire and even explosion of the battery.
An apparatus for charging lithium batteries therefore has to be able to monitor and regulate with great precision the charge voltage, maintaining it within the aforementioned limits.
From the prior art numerous devices are known that manage the entire recharging process, normally by means of an internal finite-state machine.
A problem that is common to prior-art battery chargers is that in the circuit that connects the battery to the battery charger there are resistances that introduce voltage drops that alter the charge voltage read by the battery charger, thus preventing precise monitoring of the charge voltage required by the lithium batteries. These voltage drops will be indicated henceforth as parasitic voltage drops.
This drawback is particularly marked in the case of portable devices supplied by lithium batteries, for example manual readers of encoded information (such as optical codes such as barcodes, stacked codes, two-dimensional codes, and colour codes or printed characters, stamps, logos, signatures or electronic labels for radiofrequency identification, so-called “tags”) or for acquiring images of “cordless” type, so-called “guns” for the shape of the grippable casing. In the present context the term “gun” is intended to indicate all grippable reading devices without cord (“cordless”) that communicate with a base or a remote computer by means of radio waves and those provided with a memory for storing data that are then downloaded into the base memory, which are also known by the term “batch”.
Typically, the “guns” are charged by means of an external recharging device, a so-called “cradle” that is suitable for receiving and housing the “gun” for exchanging data therewith (and thus acting as a base) and providing it with the electric power necessary for recharging. The contacts or connectors by means of which an electric connection is established between the “gun” and the “cradle” and the internal cables of the “gun” and the “cradle” have electrical resistances that causes parasitic voltage drops of significant value in the connection between the battery charger housed in the “cradle” and the battery housed in the “gun”.